Turning off the mags totally is so daft that it's unfunny
Interesting view. I have never done a 'post start' mag off check, but normally do a pre-shutdown mag off check. This is becasue as mentioned before the mags are grounded out by the key switch, to stop them creating the spark. Now if there is a fault with the earthing then switching the key to off won't ground the mags, and the engine won't stop. This is clearly dangerous, as next time the mechanic is playing around, and spins the prop for whatever reason, the engine could spark to life.
The mag check is only momentary, so won't cause any damage to the engine, you just observe that the engine is obviously stopping.
To help avoid fouling, I normally lean on the taxy, go full rich for the run-up, and if the mags are fouled, increase throttle and lean until the engine just starts to sound rough. Give it a few seconds, go full rich and bring the throttle back to specified in POH (2000, 1800 whatever).
I think its crap we're still flying around in aircraft where you can destroy the engine by misuse of the mixture, throttle and prop controls, you don't know how much fuel you have becasue the fuel guages don't really work, you can destroy an engine by shock cooling, and crash becasue some ice forms in the carb, plugs foul up for a past time, and a "new" engine will burn more oil than my 1984 Dihatsu 4Track which has done 130,000 miles!
Give me a FADEC TDi any day
EA